The time of arrival of the ions at an end plate is directly related to their M/q (mass to charge state ratio). In a typical application (10 kV acceleration and 2 m flight path) 7.2 ns separate the arrival time at the end plate of two adjacent mass ions (delta(M/q=1)) for M/q=10000 varying with the inverse square route of the mass of the ions. Due to thermal and lens effects there could be a spread of some cm in the radial extent normal to the direction of flight of the ions after traveling such distance, although they may have been emitted from a small spot. Thus, determination of the ion mass is based on a signal from the instance wherein the ions hit an extended end plane rather than a small focused spot. The efficient detection of these ions at the end plane with a time resolution sufficient to separate adjacent mass ions, even when the quantities of the adjacent ions vary, is a critical issue in the performance in any TOFMS. A typical distance in the forward direction separating traveling adjacent mass ions of M/q=10000 molecules upon their arrival at the detecting plane is 140 micrometer varying with the inverse square root of the mass. Therefore, the detecting plane has to be very flat and needs to be aligned perfectly perpendicular to the direction of flight to maintain the needed time resolution. Otherwise, there is a risk that ions with close by masses will hit the detecting surface at the same time.
A common detection scheme is to place a Micro-Channel-Plate (MCP) in the detection plane. The MCP has many channels each with diameter 5–20 microns and at an angle of 5–15 degrees to the normal. The open channels subtend 55–40% of the sensitive area of the MCP, the rest being the conducting area between the open channels. Thus, about 50% of the ions that hit the area between the channels are lost. Those impinging ions that hit an open channel can generate secondary electrons in the cannels that are further multiplied along the channel. There is some variation in timing of the initiation of the signals due to the different depth in the channel in which the ion hit the channel walls.
For the various ions that generate multiplication a train of pulses each of FWHM of 0.5–4 ns can be obtained in several ways:                By using a second MCP behind the first one followed by an anode. The train of pulses thus obtained from the anode has usually to be extracted from a high voltage level,or an additional screen at ground potential has to be introduced in front of the MCP input, so that the MCP input will be at about 2 kV, and the output at ground potential.        By accelerating the electrons from the first or second MCP towards a fast scintillating material and measuring the train of light pulses with a fast photo-multiplier tube (PMT).        
Other arrangements involve a flat plate that the ions hit and generate electrons. The electrons are collected to some area usually to be further multiplied or amplified by an electron multiplier arrangement, or further accelerated towards a scintillating material. In such an arrangement, especially if the ion detecting area is of more than 1 cm in diameter, a significant time spread in the collection time of the electrons occurs due to the different flight path of the electrons. Also, some distortion of the electric field seen by the ions may be introduced effecting their arrival time to the detecting plate. In such arrangement the time resolution is usually worse than in the MCP arrangements.
Other TOFMS employ a combination of an electric field and a magnetic field to extract the secondary electrons and bring them to an electron detector such as MCP or scintillator. A. Brunelle et al., a group from the university of Orsay, France, disclose such a TOFMS in International Journal of Mass Spectrometry and Ion Processes 126, 65–73 (1993) (hereinafter Brunelle 1) and in Rapid Communications in Mass Spectrometry 11, 353–263 (1997) (hereinafter Brunelle 2) A similar system is disclosed by a group of the University of Delaware and Dupont in H. C. Michelle Byrd and C. N. McEwen, Analytical Chemistry 72, 4568–4576 (2000) (hereinafter Bird et al.) and C. N. McEwen, S. P. Thompson and V. C. Parr, A new Detector for Polymer Characterization by MALDI-TOF Mass Spectrometry. Proceedings of the 46 ASMS Conference for Mass Spectrometry and Allied Topics, Portland, Oreg., May 12–16, 1996; p 1072.
The TOFMS according to Brunelle comprises a ring extraction electrode to extract the electrons produced by ion impingement and a magnetic field after the extraction electrode (e.g. p 356 in Brunelle 2). The ring shaped extraction electrode used in Bruelle 2 introduces severe time variation for the ions passing through it. Thus, sufficiently high time resolution for a TOFMS cannot be obtained.
The device relied upon by the group from university of Delaware and Dupont (e.g. Bird et al., p 4570) also has an electron extraction field before the magnet. The detection path of the charged particles passes through screens 3 times (incoming ion, electron into the magnetic field region, electron from the magnetic field region towards the MCP detector). The electrons are subsequently detected by means of an MCP. As discussed above an MCP provides only limited efficiency, since for MCP 45% of the electrons that reach an MCP do not generate a signal. Additionally, every screen to be passed reduces the detection efficiency by 10–20% and generates secondary particle background.